1. Field of the Invention
The present invention relates generally to media treatment processes and more specifically, to processes used to remove pathogens from water or wastewater.
2. Description of Related Art
Microorganisms pathogenic to humans are ubiquitous in the water cycle and have been found in drinking water and wells (Goyal et al., 1984; Gerba and Rose, 1990; Kramer et al., 1996). Major groups of microbial pathogens include viruses, bacteria, and protozoa. Sources of microbial contamination include, but are not limited to, leaking septic tanks and sewer lines, wastewater discharge and reuse, landfills, and sewage sludge application on land (Yates et al., 1985), as well as runoff and infiltration from animal waste-amended fields (McMurry et al., 1998). The USEPA Science Advisory Board cited drinking water contamination as one of the highest-ranking environmental risks and reported that microbial contaminants (e. g., bacteria, protozoa, and viruses) are likely to be the greatest remaining health risk management challenge for drinking-water suppliers (USEPA, 1990). Types of illness that can result from exposure to microbial pathogens range from mild or moderate cases lasting a few days to more severe infections that last several weeks and may result in death in the more sensitive subpopulations (e. g., young children, elderly, and people with compromised immune systems). A major study of the occurrence of pathogens in U.S. ground waters tested samples collected from 448 sites in 35 states for various indicators of fecal contamination (total coliform, E. coli, somatic and male-specific coliphages, and human viruses) (Abbaszadegan et al., 2003). It was found that 31.5% of the samples were positive for one or multiple pathogenic viruses using polymerase chain reaction (PCR), and human viruses were detected in 4.8% of the samples by cell culture.
Concerns over the number of waterborne disease outbreaks that continue to occur in the U.S. despite improvements in drinking water treatment practices, have resulted in the development of regulations to reduce such risks. The Surface Water Treatment Rule (SWTR) (USEPA, 1989a) and Interim Enhanced SWTR (USEPA, 1989b) were established in an effort to control microbial contaminants in drinking water systems using surface water or groundwater under direct influence of surface water. In addition, the EPA recently proposed a Ground Water Rule (GWR). The GWR is aimed at addressing microbial contamination of ground water-supplied drinking water systems in accordance with the Safe Drinking Water Act (SDWA) of 1974, as amended in 1986 and again in 1996 (USEPA, 2000). The GWR and other regulations address microbial contamination and DBP formation in drinking water systems in order to reduce public health risks resulting from pathogenic contamination and DBP toxicity. The 1986 SDWA amendments directed the EPA to establish national primary drinking water regulations requiring disinfection as treatment for the inactivation of microbiological contaminants for all public water systems, including systems supplied by ground water sources. Worldwide, there is a great interest to redirect investments in water infrastructure to cheap, decentralized, and environmentally sustainable technologies to meet the demand for water and energy in developing countries. The United Nation's Millennium Development Goal is to bring 100 million small farming families out of extreme poverty through low-cost water technologies in the next 10 years. Furthermore, technologies with greater efficiencies than chlorine or iodine to remove microbial agents from water will significantly improve the effectiveness of portable water treatment devices. In the last decade, zero-valent iron has been increasingly used as a reactive material in permeable reactive barriers (PRBs) to remediate groundwater contaminated with solvents and other organic, metallic, and radioactive chemicals.
Although viruses are only one type of microbial pathogen known to contaminate groundwater, they are much smaller than bacteria and protozoan cysts, and thus are filtered out to a much smaller extent in porous media than bacteria due to their size. Therefore viruses can travel much longer distances in the subsurface (Jin and Flury, 2002). Viruses are identified as the target organisms in the GWR because they are responsible for approximately 80% of disease outbreaks for which infectious agents were identifiable (Ryan et al., 2002). In addition to viruses, the protozoan parasite Cryptosporidium is another waterborne pathogen of significant public health concern. Survey studies have found oocysts in 4-100% of surface water samples examined, with concentrations up to 10,000 oocysts per 100 L of water. (Lisle et al., 1995). Groundwater may also contain oocysts as shown by a 22% prevalence rate in one study performed in the U.S. (Hancock et al., 1998). The difficulty in controlling cryptosporidiosis is due in part to the resistance of Cryptosporidium oocysts to commonly used levels of disinfectants in drinking and recreational waters. (Korich et al., 1990).
Disinfection is an important water treatment process for preventing the spread of infectious diseases. While mostly effective for removing many bacteria, classical disinfectants such as chlorine have been shown as not always being sufficiently effective against viruses and protozoa (Payment and Armon, 1989; Bull et al., 1990).
Data collected by the Centers for Disease Control and Prevention (CDC) and the EPA indicate that almost as many waterborne disease outbreaks were reported between 1971 and 1996 in systems with disinfection treatment that was inadequate or interrupted (134 outbreaks) as were reported in the same period among systems that did not disinfect (163 outbreaks) (USEPA, 2000). High doses of chlorine also can produce excessive amounts of disinfection by-products (DBPs) through reaction with DBP precursors such as natural organic matter in source water. More than 500 DBPs have been identified (Plewa et al., 2004). The most commonly reported, and currently regulated, chlorination DBPs include total trihalomethanes (TTHM: chloroform, bromodichloromethane, dibromochloromethane, and bromoform) and haloacetic acids (HAA5, monochloroacetic, dichloroacetic, trichloroacetic, monobromoacetic and dibromoacetic acids). Many of these DBPs are known or suspected human carcinogens and have been linked to bladder, rectal, and colon cancers (U.S. EPA, 2003a and b). Studies on human epidemiology and animal toxicology have also demonstrated links between chlorination of drinking water and reproductive and developmental effects, such as fetal losses and neural tube and heart defects (U.S. EPA 2003b). It has been estimated that about 254 million Americans are exposed to DBPs, and the U.S. EPA is proposing the Stage 2 Disinfection Byproduct Rule (U.S. EPA, 2003c), which is aimed at protecting public health from DBPs in water. Consequently, it is increasingly recognized that removal of natural organic matter during water treatment is critical for minimizing formation of DBPs in drinking water (Jarvis et al., 2005).
Although strongly oxidizing disinfectants other than chlorine, such as chloramines, ozone, and chlorine dioxide, are being used in the U.S. and Europe, and alternative non-oxidant-based disinfection methods such as ultraviolet (UV) irradiation and membrane processes are available, these options are often more expensive in terms of capital investment and operation cost and/or complex and thus difficult to implement. In addition, some of the non-chlorine disinfection alternatives also generate DBPs, which can include bromate.
In addition to drinking water treatment, wastewater discharge and reuse (e. g., through groundwater recharge and irrigation) and land-application of sewage sludge have attracted increasing public attention and growing concern because of the presence of human and animal pathogens in treated wastewater and sludge. Because wastewater treatment generally includes primary and secondary treatment, which may only remove a fraction of the pathogenic microorganisms, discharge of treated wastewater and sludge represent a potential source of microbial contamination. In addition, chlorination and dechlorination (often with sulfur dioxide or sulfite salts) of treated wastewater prior to its discharge not only adds to the treatment cost but also generates undesirable DBPs including THMs, HAAs, and N-nitrosamines that are highly toxic to aquatic organisms (Jensen and Helz, 1998; MacCrehan et al., 1998).
The Department of Homeland Security has reported that water treatment facilities that use chlorine are more attractive targets for terrorist attack. A major failure of chlorine storage tanks could produce a chlorine gas plume that would affect residents within a ten-mile radius. Currently about 600 facilities could threaten between 10,000 and 100,000 people (U.S. DHS, 2003). In addition, accidental release of chlorine gas may have catastrophic consequences. Moreover, some chlorine-manufacturing facilities still use mercury cell electrolysis, a process that can release large quantities of mercury into the environment. If a safer, non-oxidant-based disinfection method is used in a treatment facility to provide additional removal of microbial pathogens, the consumption, transport, and on-site storage of chlorine may be reduced, thus minimizing our dependence on chlorine and the risks associated with the chlorine infrastructure.
One of the most complex problems facing the water industry today is how to provide adequate protection against infectious diseases without the risk from disinfectants and DBPs. It is difficult to manage both microbial and DBP risks, and even more challenging to do so at an acceptable cost. With increasing population and growing demand for potable water, increasingly stringent environmental regulations, and heightened security concerns, developing innovative, inexpensive, and robust technologies that can simultaneously reduce the risks of pathogens, DBPs, and residual disinfectants in drinking water is of utmost urgency.
Portable drinking water systems or chemical additives are available for household use, traveling to remote areas including earthbound and outer space, recreation including camping and hiking, humanitarian purposes, military and engineering operations in remote areas, and disaster relief where water supplies are interrupted. Effective additives for pathogen removal that are currently used in those devices include chlorine, chlorine dioxide, and iodine. However, although chlorine and iodine are effective for removal of bacteria, they are limited in effectiveness against viruses and protozoa (e.g. Cryptosporidium and Giardia.)
In the last decade, elemental iron (a. k. a. zero-valent iron, metallic iron, Fe(0), and iron metal) has been increasingly used as a reactive material in permeable reactive barriers (PRBs) to remediate groundwater contaminated with solvents and other organic, metallic, and radioactive chemicals (Vidic, 2001; EPA 2002a). PRBs are subsurface treatment zones that contain reactive materials, such as elemental iron, placed in the flow path of contaminated groundwater. PRBs have higher permeability than adjacent aquifer materials and, as groundwater flows through the PRB, dissolved contaminants are removed from water through physical and chemical processes such as adsorption, reduction reactions, and precipitation. Since 1995, more than 120 field-scale PRBs have been installed worldwide, most of them in the U.S. and Europe (RTDF, 2003; ETI, 2005).
Approximately 80% of the PRBs contain elemental iron, typically in the form of inexpensive commercial iron filings (EPA, 2002a). Unlike the conventional pump-and-treat method, PRBs are in situ and passive and involve minimal maintenance and operation costs. Iron PRBs also have long service lives and have been shown to remove and/or degrade pollutants effectively and continuously for multiple years (EPA, 2002a, b).
In addition to its use in PRBs for groundwater remediation, iron was evaluated for water and wastewater treatment in recent years. It has been shown that elemental iron could be used to treat wastewaters containing refractory compounds such as azodyes, nitroaromatic compounds, and explosives (Perey et al., 2002; Oh et al., 2003). It has also been demonstrated that reductive treatment with iron rapidly converts certain refractory compounds into products that are much more degradable in the subsequent chemical or biological oxidation processes (Perey et al., 2002; Oh et al., 2003).
Furthermore, elemental iron has also been shown to remove arsenic and other chemical pollutants from water (Farrell et al., 2001; Melitas et al., 2002). These authors reported that corrosion of iron continuously generates iron oxides to adsorb and remove arsenic from water. In December 2003, Sengupta of Lehigh University reported that the use of polymeric ion exchange beads impregnated with ferric hydroxide could be used to remove arsenic from well water in India.
The present invention addresses above-described problems of biological agents and DBPs in water and provides solutions thereto.